Because for many children, this kind of response will lead to a coercive cycle.
And many parents will interpret this advice to mean that they should use a shouting, threatening tone themselves and allow emotion and body language which is big and intimidating and threatening, even if they fall short of actual violence, or they will get drawn into "tit for tat" style argument/slanging match and end up in an out of control place themself, which helps nobody.
By responding in a "big and angry" way, the intent is (intentionally or not) to be a bigger threat, to overpower, to show the child that you actually are the bigger threat and they should back down or else. It's similar to the whole alpha/pack hierarchy thing. And it will make some children back down, I'm not denying that. It will genuinely intimidate them into compliance. And perhaps if they were trying it on and they do actually have the skills/ability to control their own behaviour this kind of thing is enough of a deterrent to stop them in their tracks and make them much less likely to try it again (though they may well go on to divert that energy onto younger or smaller targets such as smaller/weaker children, or animals, having seen this modelled). And if this is infrequent, the home environment generally has good structure and role models and there is enough warmth to the relationship generally, it can be repaired and not cause much of a rupture in general to the entire relationship, which is probably why it feels like reasonable advice to give.
But for some children, when the adult escalates the response, the child escalates their response too, the adult then feels forced to escalate theirs higher and the child escalates too and before you know it the adult is at a point where they are using violence or utterly terrorising the child to a point which is not OK. Or they give up and leave before they do something they regret. This is called the coercive cycle and it is highly destructive, leading on both sides to child abuse and to worsening of the child's behaviour (it's considered a major route to ODD) because it is extremely unintentionally rewarding for both parties, based on the slot machine principle of behaviour training (unpredictable rewards are more rewarding than consistent ones). It feels counterintuitive, because nobody classes having a massive violent screaming argument with their child/parent as fun, but in behavioural training, reward does not necessarily = fun - the main reason that both parent and child will continue to repeat this response is that it works sometimes. Sometimes, the parent succeeds in intimidating the child into compliance, sometimes the child succeeds in retaliating until they escape the original demand. And nobody is learning more civilised ways of dealing with disagreement or difficulty/not wanting to do something, meaning the response of escalation is getting more entrenched and automatic. Which is another reason why it tends to repeat, especially if the child has underlying issues such as neurodivergence or trauma meaning that they may encounter difficulty with rules/behaviour expectations more than other children, or especially if the relationship between the parent and child is poor meaning they are more likely to interpret neutral communication negatively than positively.
It also becomes more difficult to physically intimidate a child into compliance as they get older, and particularly for mothers, they may find that their sons overpower them in strength and sometimes height by around 11 years old. So it is not a sustainable strategy if a child has significant problems where violence and aggression are present and likely to continue.
However I feel there is potential for misunderstanding on the opposite side as well, so thank you for highlighting that.
The word calm is about the parent being in control, not about being passive or gentle. Calm refers to the voice tone being steady and unemotional, and the adult's responses being predictable and consistent rather than unpredictable and threatening.
Strong and firm - absolutely yes. But the strong should ideally be more in the direction of "I'm not bothered by this. I don't see you as a threat. You can't hurt me, because I won't let you. This is not the way we deal with big feelings." And also of being in control of yourself and your own emotions/responses, rather than "I am bigger and stronger, so don't you dare, you'll regret it"
Agree that words do not especially help - when a child is in such a riled up state that they are being physically aggressive, they don't have much access to the brain centres which deal with language processing anyway. They need less words, more action. The action should basically be removing any objects of potential harm such as the knife, and if necessary moving the child away from the situation or to a calmer/quieter place, and then removing your own attention unless you can keep yourself very steady and calm, because this provides a co-regulating effect. It's not actually true that being calm only helps compliant children. Children who are compliant probably already have very good self-regulation abilities. (Or sometimes they have gone into shutdown/fawn but that is for another post). Children who are becoming aggressive and violent categorically do not have good self-regulating abilities and co-regulation is a major way that we learn this skill as humans. I know this sounds very fluffy and airy fairy, and the word "co-regulation" is misused all over social media at the moment but there is a good evidence base for the actual researched concept. And no, co-regulation does not mean being a punchbag for a violent child, and it does not mean stuffing your own emotions down and plastering on a fake calm face while your heart is going 180bpm in your chest. If you can't genuinely self-regulate (and it is extremely hard to do when somebody is throwing things at you!) enough to be the co-regulation "model" then it is better to take some space instead. Often someone in a violent state will start to calm down when their audience is removed, and usually this will progress to a point where you can as the parent go in and be a co-regulating influence.
But in that moment, he needs to know who is in charge, and he needs to know that the rule is absolute with no exceptions, and physical violence will not be tolerated.
Completely agree with this. I think you can do this while being calm in the way I have described. Agree that "calm" probably needs more qualifying, though. I'll bear that in mind for the future 